Oogenesis, Ovulation, Fertilization & Implantation (MED 1112)

7/23/2019 Oogenesis, Ovulation, Fertilization & Implantation (MED 1112)

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General Embryology

Oogenesis,

Ovulation, Fertilization and Implantation

Prof Dr.N.Jeyaseelan

Faculty of Medicine

SEGi University.

MED:1112General Embryology


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Learning outcome

At the end of this session, the student should be able to,

1.Explain oogenesis, ovulation and corpus luteum.

2.Describe the capacitation and acrosome reaction.

3. Define fertilization and discuss the three phases of fertilization.

4.Explain the results of fertilization.

5.Explain cleavage, inner cell mass, outer cell mass, blastocyst and

implantation.

6.Clinical correlates Discuss middle pain, infertility in females, in vitrofertilization, test tube baby, Gamete intrafallopian transfer (GIFT),Ectopic pregnancy, tubal ligation.


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What is Oogenesis?

Oogenesis is the formation and development of

the ovum.

Primordial germ cells in the gonad of a genetic

female differentiate into oogonia

(Fig.1).

These cells undergo mitotic divisions and some of

them differentiate into primary oocytes (Fig.1).

Oogenesis


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Fig.1 Differentiation of primordial germ cells into oogonia


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The secondary oocyte divides to give rise to

one mature oocyte and one polar body (Fig.2,3).

1

st

polar body divides to give rise to two polar

bodies

(Fig.2).

Primary oocytes give rise to secondary oocyte

and 1

st

polar body (Fig.2).


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Fig. 2 A Primary oocyte produces only one mature gamete.


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Fig.3 Maturation of the oocyte


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Ovulation is the discharge of the oocyte from the

ovary (Fig.3).

The oocyte is discharged with its cumulusoophorus cells (Fig 4).

At this stage the 1st meiotic division is completed

and the secondary oocyte has started its 2

nd

meiotic

division .

Ovulation


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Fig. 4 Ovulation

Note the relationship of fimbriae of uterine tube during ovulation


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Primary oocytes remain in prophase anddo not finish their first meiotic divisionbefore puberty is reached.

With the onset of puberty the primordialfollicles develop into mature follicles and

the primary oocytes complete their firstmeiotic division.


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What is puberty?

Puberty is the sequence of events by which achild is transformed into young adult.

Gametogenesis (in males) oogenesis (infemales) begin as well as secretion of gonadalhormones.

Growth of secondary sexual characters anddevelopment of reproductive functions.


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Ages of presumptive puberty

12 years in girls

14 years in boys


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Immediately preceding ovulation theGraafian follicle increases rapidly in size.

This increase in size is under the influenceof FSH and LH.

Under the influence of FSH the primordial

follicle matures into the Graafian follicle (Fig. 5).


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Fig.5 Primordial follicle (A) matures into the Graafian follicle (C)


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The oocyte remains a primary oocyteuntil shortly before ovulation.

During ovulation the fimbriae of theovary sweep over the rupturing folliclecollecting the oocyte and guiding it into

the uterine tube (Fig.4).


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Corpus luteum

Following ovulation remaining granulosa cells in

the wall of the ruptured follicle alongwith the cells

from the theca interna (Fig.6) are getting vascularised

and become polyhedral.

Under the influence of the luteinizing hormone

these cells develop a yellow pigment and change into

luteal cells.

These luteal cells form the corpus luteum.

Corpus luteum secrete progesterone.


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Fig.6 A Graafian follicle just before ruptureB- Ovulation

C

The Corpus luteum


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Transport of oocyte

Once the oocyte is in the uterine tube it ispushed toward the lumen of the uterus bycontractions of the muscular wall .

Fertilized oocyte reaches the uterine lumen inapproximately 3 4 days (Fig.7).


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A Ovary

B

Uterine tube

(Fallopian tube)

C Uterine lumen

D - Vagina

Fig.7 Parts of female genital system


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Fertilization

It is a process by which male and femalegametes fuse.

It occurs in the ampulla of the uterine tube.

Ampulla is the widest part of the uterine tube(Fig.8).


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Fig. 8 Uterine tube

Note the ampulla of uterine tube


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Spermatozoa and the oocyte remain viable in

the female reproductive tract for approximately

24 hours.

The ascent of spermatozoa in the female

genital tract is caused by the contractions of the

musculature of the uterus and uterine tube.


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For fertilising the oocyte the spermatozoa must

undergo,

1. Capacitation.

2. Acrosome reaction.


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1. Capacitation

It is a period of conditioning in the femalereproductive tract that lasts approximately 7hours.

During this time a glycoprotein coat andseminal plasma proteins are removed fromthe plasma membrane that overlies theacrosomal region of spermatozoa.

Only capacitated sperm can pass through thecorona cells and undergo acrosome reaction.


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2 .Acrosome reaction

This reaction culminates in the release of enzymesneeded to penetrate the zona pellucida.

The three phases of fertilization include,

1. Penetration of corona radiata.2. Penetration of zona pellucida.

3. Fusion of oocyte and sperm cell membranes.


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1.

enetration of corona radiata (Fig.9).

200

300 million spermatozoa are deposited in the

female genital tract .

Only 300 500 reach the fertilization site.

Only one is needed for fertilization.


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2. Penetration of zona pellucida (Fig.9).

Release of acrosomal enzymes allows the sperm

to penetrate the zona.

Only one spermatozoa seems to be able to

penetrate the oocyte (Fig.10).


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3. Fusion of oocyte and sperm cell membranes

Once a sperm has entered the oocyte, the

oocyte membrane becomes impenetrable to

other spermatozoa thereby preventing

polyspermy.


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Fig. 9 Three phases of oocyte penetration


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Fig.10 Stages from ovulation to two-cell stage.


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The oocyte finishes its 2nd meiotic division

immediately after entry of the spermatozoon.

Its chromosomes 22 + X become arranged in a

vesicular nucleus known as the female pronucleus

(Fig.10).


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Meanwhile the spermatozoon moves forward

until it lies in close proximity to the female

pronucleus.

Its nucleus becomes swollen and forms the

male pronucleus (Fig.10).


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The results of fertilization are,

1. Restoration of diploid number ofchromosomes, half from the father and halffrom the mother.

2. Determination of the sex of the newindividual.

3 . Initiation of cleavage.


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Cleavage

Once the zygote has reached a two-cell stage it

undergoes a series of mitotic divisions.

This results in an increase in cell number.


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Blastomeres

The cells which become smaller with each

cleavage division are known as Blastomeres.

Approximately 3 days after fertilization the cells

divide again to form a 16 - cell Morula (Fig 11).


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Fig. 11 Development of zygote from two-cell stage to Morula stage.


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Inner cells of the Morula constitute the Inner cell

mass while the surrounding cells compose the outercell mass (Fig.12).

The inner cell mass give rise to the tissues of theembryo proper .

The outer cell mass forms the trophoblast whichcontributes to the placenta.


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Fig.12 Human blastocyst showing inner cell mass trophoblast cells


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Blastocyst

By the time the morula enters the uterine

cavity the intercellular spaces become confluent

and a single cavity the blastocele is formed.

At this time the embryo is known as the

blastocyst (Fig.12).

The cells of the inner cell mass is now referred

as the embryoblast while those of the outer cell

mass is the trophoblast (Fig.12).


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Implantation

Attachment of the fertilized ovum (blastocyst)

to the endometrium of uterus and its subsequent

embedding in the compact layer.

It occurs six or seven days after fertilization of

the ovum.


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At the time of implantation the mucosa of the

uterus is in secretory phase (Fig.13).

Three layers in the uterine endometrium can berecognised . Compact layer (superficial) spongylayer (intermediate) and a basal layer.


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At the eighth day of development, the blastocyst

is partially embedded in the endometrial stroma

(Fig.13).

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Fig.13 Events taking place during1stweekof development


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Clinical Correlates

1. In some women, ovulation is accompanied by slight

pain, known as middle pain and this eventnormally occurs near the middle of the menstrualcycle.

2. Ovulation is generally accompanied by a rise in basaltemperature, an event that can be monitored indetermining when release of the oocyte occurs.

3. Some women fail to ovulate due to diminishedconcentration of gonadotropin.

4. Fertilization can be prevented by a variety ofcontraceptive methods.


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5. Infertility in females may be due to number of

causes including occluded oviducts and absence of

ovulation.

Infertility is the inability of a couple to become

pregnant after 1 year of unprotected sexual

intercourse using no birth control methods.


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6. In vitro fertilization (IVF).

IVF a

mans sperm and a womans egg are

combined in a laboratory dish, where fertilization

occurs.

The resulting embryo is then transferred to the

womans uterus

to implant and develop naturally.

The term

Test tube baby

is often used to refer

to children conceived with this technique.


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orlds first test tube baby

(Louise Brown)


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7. Gamete intrafallopian transfer (GIFT)

This technique is introducing oocytes and

sperm into the ampulla of the fallopian tube

where fertilization takes place (Fig.14).

Fig. 14 Gamete intrafallopian transfer


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8. Ectopic pregnancy

Implantation and growth of the fertilised ovum

may occur outside the uterine cavity in the wall of

the fallopian tube (Fig.15).

Tubal abortion or rupture of the tube, witheffusion of a large quantity of blood into theperitoneal cavity, is the common result.

Fig 15. Ectopic Pregnancy


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g


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9. Tubal ligation

Ligation and division of the uterine tubes is a

method of obtaining permanent birth control

(Fig.16).

Fig. 16 Tubal ligation


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g


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Reference Book

1. Langman's Medical Embryology 12th ed. - T.

Sadler (Lippincott, 2012).


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Oogenesis, Ovulation, Fertilization & Implantation (MED 1112)

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